I didn't mind Ezri. . . She was cute and a bit shy, which happens to be a favored character type for me.
The problem is she was a replacement to Jadzia who was the sex-symbol of DS9 and you just can't replace that type of character with a cute and shy type like Ezri without backlash.
Jake did serve a purpose on the series. It just wasn't a purpose that had much of anything to do with the overall story.
All characters on a TV show have a purpose. In the case of Jake Sisko it was to try and draw a teen audience by giving them a character they might be able to identify with, but he offered no real addition to the story. It was the same case with Wesley, but the big difference is that they set him up as a savior in TNG who could solve problems that not even the experienced adults could, which of course rubbed the adult audience the wrong way.
I'm so glad they killed that formula of inserting a teenage character in the series after DS9.
That being said. . . My least favorite character of DS9 is definitely Jake.
I was never very fond of Jadzia Dax, she often seemed fake / poorly acted to me and something of a drama queen. I thought Ezri was a better Dax than her as far as acting and seeming like a real person in the show.